Phenol is an important product in the chemical industry. For example, phenol is useful in the production of phenolic resins, bisphenol A, ε-caprolactam, adipic acid, alkyl phenols, and plasticizers.
Currently, the most common route for the production of phenol is the Hock process. This is a three-step process in which the first step involves alkylation of benzene with propylene to produce cumene, followed by oxidation of the cumene to the corresponding hydroperoxide and then cleavage of the cumene hydroperoxide. The product comprises equimolar amounts of phenol and acetone. However, the world demand for phenol is growing more rapidly than that for acetone. In addition, due to developing shortages in supply, the cost of propylene is likely to increase.
Thus, a process that avoids or reduces the use propylene as a feed and coproduces higher ketones, such as methyl ethyl ketone and/or cyclohexanone, rather than acetone may be an attractive alternative route to the production of phenol. For example, methyl ethyl ketone is in demand for use as a lacquer and a solvent and for dewaxing of lubricating oils. In addition, there is a growing market for cyclohexanone, which is used as an industrial solvent, as an activator in oxidation reactions and in the production of adipic acid, cyclohexanone resins, cyclohexanone oxime, caprolactam and nylon 6.
It is known that phenol and methyl ethyl ketone can be co-produced by a variation of the Hock process in which sec-butylbenzene is oxidized to obtain sec-butylbenzene hydroperoxide and the hydroperoxide is decomposed to the desired phenol and methyl ethyl ketone. The sec-butylbenzene can be produced by alkylation of benzene with linear butenes over zeolite beta or a molecular sieve of the MCM-22 family. Details of such a process can be found in, for example, International Patent Publication No. WO2006/015826.
Similarly, U.S. Pat. No. 6,037,513 discloses that cyclohexylbenzene can be produced by contacting benzene with hydrogen in the presence of a bifunctional catalyst comprising a molecular sieve of the MCM-22 family and at least one hydrogenation metal selected from palladium, ruthenium, nickel, cobalt and mixtures thereof. The '513 patent also discloses that the resultant cyclohexylbenzene can be oxidized to the corresponding hydroperoxide and the peroxide decomposed to the desired phenol and cyclohexanone.
However, the production of phenol using sec-butylbenzene and/or cyclohexylbenzene as the alkylbenzene precursor is accompanied by certain problems which either are not present or are less severe with a cumene-based process. For example, in comparison to cumene, oxidation of sec-butylbenzene and cyclohexylbenzene to the corresponding hydroperoxide is very slow in the absence of a catalyst and is very sensitive to the presence of impurities. As a result, U.S. Pat. Nos. 6,720,462 and 6,852,893 have proposed the use of cyclic imides, such as N-hydroxyphthalimide, as catalysts to facilitate the oxidation of alkylbenzenes, such as sec-butylbenzene and cyclohexylbenzene.
With regard to the hydroperoxide cleavage step, current commercial phenol/acetone processes almost exclusively use a sulfuric acid catalyst, despite the fact that this yields phenol selectivities of only 92 to 96% of theoretical. The most common side reactions in sulfuric acid catalyzed cumene hydroperoxide cleavage include: 1) dehydration of carbinols (by-product from oxidation) forming α-methylstyrene, which can alkylate phenol forming heavy products and reducing the yield of phenol; 2) aldol condensation of ketone reducing ketone yield; and 3) oligomerization of olefins forming oligomers, all of which contribute to high boiling residue (“phenol tar”) formation in the final product separate step. As a result, cumene hydroperoxide cleavage is generally carried out in multiple steps to reduce “phenol tar” formation. In addition, the sulfuric acid has to be properly neutralized after the cleavage step to avoid further reactions of the cleavage products.
All of these problems increase the complexity and investment involved in the cleavage process and hence various alternatives to sulfuric acid have been proposed for the production of phenol from cumene hydroperoxide. For example, other homogeneous acid catalysts, such as perchloric acid, phosphoric acid, toluenesulfonic acid and SO2, have also been shown to be effective. However, all of these homogeneous catalysts suffer from the same downstream acid neutralization and product purification problems as sulfuric acid. To minimize these problems, various solid acid catalysts have been proposed for the heterogeneous cleavage of cumene hydroperoxide. For example, U.S. Pat. No. 4,490,565 discloses the use of zeolite beta in the cleavage of cumene hydroperoxide, whereas U.S. Pat. No. 4,490,566 discloses the use of a Constraint Index 1-12 zeolite, such as ZSM-5, and EP-A-492807 discloses the use of faujasite in the same process. The use of smectite clays in the acid-catalyzed decomposition of cumene hydroperoxide is described in U.S. Pat. No. 4,870,217.
U.S. Pat. No. 4,898,995 discloses a process for the coproduction of phenol and acetone by reacting cumene hydroperoxide over a heterogeneous catalyst consisting of either an ion exchange resin having sulfonic acid functionality or a heteropoly acid, such as 12-tungstophosphoric acid, on an inert support, such as silica, alumina, titania and zirconia. Such heteropoly acid catalysts are generally used as their hydrates, and as such are inherently unstable at temperatures in excess of 350° C.
U.S. Pat. No. 6,169,215 discloses process for producing phenol and acetone from cumene hydroperoxide, wherein said process comprises the step of contacting cumene hydroperoxide with a solid-acid catalyst produced by calcining a source of a Group IVB metal oxide with a source of an oxyanion of a Group VIB metal at a temperature of at least 400° C. The Group IVB metal oxide is selected from zirconia and titania and the Group VIB metal oxyanion is selected from oxyanions of chromium, molybdenum and tungsten.
In the case of the production of phenol from other alkylbenzenes, such as sec-butylbenzene and/or cyclohexylbenzene, to date little research has been conducted on the hydroperoxide cleavage step, although most proposals focus on the use of sulfuric acid and similar homogeneous catalysts. It is, however, apparent that any viable cleavage method will have to address the fact that production of the hydroperoxide is likely to require the use of a catalyst, such as a cyclic imide, and hence the direct product of the oxidation step could well contain nitrogen compounds, which are known poisons for the acid catalysts typically used for the cleavage step.
According to the present invention, it has now been found that certain mixed metal oxides are highly active catalysts for the cleavage of the hydroperoxides of higher alkylbenzenes and are capable of producing phenol with selectivities of 98% and higher. Moreover, although it may be desirable to remove the nitrogen impurities resulting from the catalyst used in producing the hydroperoxide, it has also been found that poisoning of the catalyst by such nitrogen impurities can be mitigated by diluting the hydroperoxide with a polar solvent and that the poisoned catalyst can be effectively rejuvenated by washing with a polar solvent. In addition, since the catalyst is a solid, the downstream neutralization and purification problems inherent with homogeneous catalysts, such as sulfuric acid, are avoided.